Final answer:
The oldest metabolic pathway is glycolysis, followed by anoxygenic and oxygenic photosynthesis. The oxygen-rich atmosphere led to the evolution of the Krebs cycle and the electron transport chain.
Step-by-step explanation:
The evolutionary sequence of metabolic processes generally starts with glycolysis, which is thought to be the oldest metabolic pathway. Glycolysis is the process by which glucose is broken down to pyruvate, producing ATP anaerobically.
This pathway was present before oxygen was abundant in the atmosphere and doesn't require oxygen, indicating its primordial origin.
After glycolysis, the next processes to evolve were likely forms of photosynthesis. The initial form was anoxygenic photosynthesis, which does not produce oxygen as a byproduct.
Once anoxygenic photosynthetic organisms contributed to the rise of atmospheric oxygen, oxygenic photosynthesis evolved. This process is what most plants and algae use today, producing oxygen as a byproduct.
With an oxygen-rich atmosphere, organisms developed pathways to utilize oxygen for more efficient energy production, leading to the evolution of the Krebs cycle (also known as the citric acid cycle or TCA cycle) and the electron transport chain (ETC).
The Krebs cycle processes pyruvate from glycolysis and produces molecules that carry electrons to the ETC. In turn, the ETC generates a large amount of ATP through oxidative phosphorylation.